[dashboard] Analytics: Measure time to page loaded

[geropl]

During our last team sync we discussed it'd help to identify slow dashboard pages if we would not only know which pages was loaded, but also when it did finish rendering.

We could try to:

• find a generic way to derive it from the browser
• find a generic way to measure it in react
• offer an explicit methods which we hook into react callbacks

\cc @AlexTugarev @gtsiolis bc you had some ideas how to implement it

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.